- Email: [email protected]
The BaBar electromagnetic calorimeter
Nuclear Instruments and Methods in Physics Research A 494 (2002) 303–307
The BaBar electromagnetic calorimeter Bernd Lewandowski Institute for Experimental Physics 1, Ruhr-University, Universitaetstrasse 150, D-44780 Bochum, Germany For the BaBar EMC Group
Abstract The BaBar electromagnetic calorimeter is a hermetic, total-absorption array of CsI(Tl)-crystals, operated at the asymmetric e eþ -collider PEP-II at SLAC. The design and the status of the performance as of February 2002 is presented. r 2002 Elsevier Science B.V. All rights reserved.
1. Requirements The BaBar electromagnetic calorimeter is used to measure electromagnetic showers over a wide energy range. The lower boundary is given at the order of 20 MeV to ensure an efficient reconstruction of B-decays containing multiple p0 s; while the upper limit is given by electrons from Bhabha events at 9 GeV; which is increased to 13 GeV to allow for possible running at the Uð5sÞ resonance. The efficient reconstruction of extremely rare decays (e.g. B0 -p0 p0 ) requires an energy resolution at the order 1% [1]. While the p0 mass resolution at lower energies ðo2 GeVÞ is dominated by the energy resolution, at higher energies an angular resolution of a few milliradians is required. The EMC is also the primary subdetector for the separation of electrons and hadrons, which requires a purity at the 0:1% level. To ensure a high efficiency in data taking over the
E-mail address: [email protected] (B. Lewandowski).
10 years lifetime of the experiment, all inaccessible detector elements must be highly reliable.
2. Implementation 2.1. Crystals The EMC is realized as hermetic, total-absorption array of finely segmented scintillating crystals [2]. The material of choice to achieve the design requirements is CsI, which is doped with thallium at the 0:1% level. The high light yield (50,000 g=MeVÞ and small Molie" re radius ð3:8 cmÞ together with the short radiation length ð1:85 cmÞ allow for a relatively compact design of a calorimeter with the required performance. The peak emission at lmax ¼ 565 nm permits the readout of the scintillation light with photodiodes, which allows the EMC to be located completely inside of the solenoid volume to minimize material in front of the crystals. To achieve the required resolution over the whole energy range, the crystals had to pass several steps of quality control
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 1 4 8 4 - 5
B. Lewandowski / Nuclear Instruments and Methods in Physics Research A 494 (2002) 303–307
304
like measurements of light yield and the independence of the light yield from the location of the scintillation along the length of the crystal. 2.2. Geometry The calorimeter consists of a cylindrical barrel and a conical endcap in forward direction, since it is operated at an asymmetric collider. The barrel is segmented in 48 axially symmetric rings of 120 crystals each, while the endcap has 8 rings with 80 crystal in the innermost rings and up to 120 crystals in the outer rings (see Fig. 1). The trapezoidally shaped crystals have a transverse size comparable to the Molie" re radius and a length increasing from 29:6 cm in the backward region to 32:4 cm in forward direction to minimize the shower leakage from increasingly higher energy particles. The 6580 crystals cover a polar angle from 15:81 to 141:81 which corresponds to a solid angle coverage of 90%: 2.3. Readout The scintillation light of the CsI(Tl) crystals is read out with two silicon PIN diodes (Hamamatsu S2744-08) glued to the backward side with a transparent polystyrene coupling plate in between using an optical epoxy to maximize light transmission. The surrounding area of the backside is covered with a white reflective plate to minimize
light loss. The crystals are wrapped in two 165 mm thick layers of white diffuse reflector (TYVEK), 25 mm aluminum foil and 13 mm Mylar foil for insulation. The aluminum housing for a pair of preamplifiers directly connected to the photodiodes is mounted on the back side, connected to the aluminum foil using an electrically conductive glue to provide a Faraday shield. To assure the reliability of all components for the 10-year lifetime, extensive studies like aging tests of the gluejoints have been done and each assembly step was accompanied by several QC tests. 2.4. Electronics The readout electronics for the BaBar EMC is located in three different sections. The first one is the In-Detector region containing all parts, which will be inaccessible after the installation of the calorimeter into the BaBar detector. Each photodiode is read out with a charge-sensitive, low noise preamplifier realized as full custom ASIC design. The signal shaping is implemented as a three stage filter CR-RC-RC with shaping times of 800–250– 250 ns: To achieve the required energy resolution, the electronics is realized as a multi range system. Therefore each preamplifier provides 1 and 32 gain outputs for each diode A and B per crystal. The main amplification and the digitization electronics takes place on the ADC-boards (ADB), which are located in the On-Detector 2359
1555
1375 920
2295
1127
38.2˚
1801
558
Interaction Point
External Support
1979
26.8˚
22.7˚
15.8˚
1-2001 8572A03
Fig. 1. Top half cross-section of the EMC, the detector is axially symmetric around the z-axis (all dimensions in mm).
B. Lewandowski / Nuclear Instruments and Methods in Physics Research A 494 (2002) 303–307
305
region being accessible for system maintenance. On the ADB the Custom Auto Range Encoding Chip (CARE) combines the two signals to an average of A and B for both gains before further amplification to provide four signals of gain 1; 4; 32; 256: Using a comparator circuit, the signal range is identified and encoded in two range bits. The corresponding signal is automatically selected with an analog multiplexer and input into a 10-bit 3:7 MHz ADC. This encoding scheme reduces the dynamic range on the data transmission from 18-bit to 13-bit. On the IO-Board (IOB) the digital data of 24 channels are send across a 30 m long optical fiber to the Read Out Modules (ROM) located in the Off-Detector region, where the continuously digitized data stream is analyzed by the signal processing algorithm called Feature-Extraction (FEX).
backs. The On-Detector electronics is cooled with chilled water pumped through channels milled into the end flanges.
2.5. Mechanical integration
The signal processing is done by an algorithm FEX. First the pedestal subtraction and the correction constants from the electronics calibration are applied to each sample. The information about pulse height and position were extracted from a polynomial fit to the peak region. This has been improved by using a digital filtering. The frequency decomposition for a typical pulse and the corresponding noise spectrum are measured to derive an optimum set of weights that maximizes the signal-to-noise ratio. This implementation has improved the equivalent noise energy for the electronics noise from 440 to 270 keV:
To minimize the material between and in front of the crystals, they are housed in ‘‘eggcrate’’ structures made of thin walled (300 mm) carbonfiber epoxy composite (CFC). The barrel structure is fabricated in 280 separate modules of 7 3y f crystals. After installation of the preamplifier housings on top of the inserted crystals, these aluminum cases are glued to the carbon fiber walls using a thermally conductive epoxy to dissipate the 100 mW load from the preamplifiers. The entire module is equipped with an aluminum strongback, which is also thermally connected. This strong-back allows the handling of the 100 kg package in all directions and is used to mount and align the 280 modules in the main support structure. This aluminum support cylinder carries in the end-flanges in forward and in backward direction minicrates, which are housing the OnDetector electronics. The inner and the outer diameter of the barrel are RF shielded with a double layer of aluminum plates. They also provide a gas seal, allowing the slightly hygroscopic crystals to reside in a dry nitrogen atmosphere. To keep the temperature inside the calorimeter stable at 2070:51C; an active FLOURINERT cooling uses pipes running along the barrel, which are connected to the strong
3. Signal processing and calibration 3.1. Electronics calibration The electronics is calibrated for non-linearities in the front end electronics. A well-defined charge is injected into the input of the preamps, which is done for all four gain ranges separately with overlaps in between to match the ranges. For this procedure, each gain is selected and the range switching disabled. 3.2. Feature extraction
3.3. Source calibration The translation of the measured scintillation light to the deposited energy requires an absolute energy calibration. The single crystal calibration is done at two energies at opposite ends of the dynamic range, requiring one single point o10 MeV at the low-energy side. Therefore, a radioactive source has to be placed in front of each crystal. For the BaBar EMC a fluid source calibration system has been realized. The fluid FLOURINERT is activated in a bath using a neutron generator producing 4 108 n=s: These neutrons are used to activate the isotope 19 F: Using the decay chain 19 F-16 N þ a; 16 N-16 On þ b;
B. Lewandowski / Nuclear Instruments and Methods in Physics Research A 494 (2002) 303–307
On -16 O þ g; 6:13 MeV photons are produced. The activated fluid is circulating through thin manifolds, which are placed between the DIRC and the EMC. The half-life of 7 s of the activated 16 N prevents a long-term activation of this calibration system, but still provides a typical photon rate of 40 Hz=crystal: At this rate a 30 min run performed weekly is sufficient to keep track of changes in the light yield due to radiation damage. 16
4. Monitoring 4.1. Lightpulser A Lightpulser system is used to monitor the complete readout chain of the EMC. The light of a xenon flashlamp (Hamamatsu L4633) is spectrally filtered to match the spectrum of CsI(Tl) scintillation light and distributed to the single crystals through a system of optical fibers. A neutral filter system also allows a continuous variation of the light intensity being used for a linearity check of the complete readout chain. Any pulse-to-pulse variation of the lamp is monitored with a precision reference system. The monitoring is performed daily as a 5 min run including validation. This fast operation guarantees a minimum disruption of data taking.
3.4. Physics events Since the light yield of a crystal is not uniform along its length, calibration points at higher energies are also required. The well-known kinematics of the Bhabha scattering is used for a single crystal calibration. Depending on the polar angle, energy points in the range from 3–9 GeV are available. One day of running at the design luminosity of PEP-II ðL ¼ 3 1033 cm2 =s2 Þ delivers approximately 200 hits per crystal. This is sufficient to realize a calibration with a statistical precision of E0:35%: The p0 calibration is used to correct cluster energies for energy losses from leakage and from interaction with the material surrounding the crystals. The used correction function is a polynomial in lnðEÞ and cosðyÞ; which provides typical correction of the order 6%:
4.2. Radiation damage The accumulated dose of radiation is measured by realtime integrating dosimeters (RadFETs), which are placed in front of the crystals. The size of the observed light loss due to radiation damage (see Fig. 2) is close to expectations, which are based on extensive irradiation tests. Start
Gain HIstory
0 -0.01
450
-0.02 Endcap
-0.03 ∆Gain / Gain
300 Barrel Backward
200
Forward Barrel
10
20 30 40 50 60 Luminosity (fb-1)
70
80
(b)
Jan 2003
0
Endcap Jul 2002
-0.1
Jan 2002
(a)
Backward Barrel
-0.09
Jul 2001
50
-0.08
Jul 2000
100 0
-0.06 -0.07
Barrel Forward
150
-0.05
Jan 2000
250
-0.04
Jan 1999
Dose [Rad]
350
Jul 1999
400
Jan 2001
306
Fig. 2. Radiation monitoring: (a) integrated dose vs. integrated luminosity measured by RadFet’s placed in front of the crystals, (b) relative gain change as measure for radiation damage of the CsI(Tl) crystals.
B. Lewandowski / Nuclear Instruments and Methods in Physics Research A 494 (2002) 303–307 π0 → γγ η → γγ Bhabhas χ c → J/ψ γ radioakt. Source MonteCarlo+BG MonteCarlo
0.06
Delta E / E
0.05
0.014
0.04 0.03
σ (E)/E = σ1/E
0.02
σ 1 = (2.30 ± 0.03 ± 0.3)%
0.01
1/4
⊕ σ2
(a)
η → γγ
σ 1 = (4.16 ± 0.04) mrad
MonteCarlo+BG
σ 2 = (0.00 ± 0.00) mrad
0.01
MonteCarlo
0.008 0.006
0 Eγ / GeV
π0 → γγ
+ σ2
0.002
0 10-1
1/2
0.004
σ 2 = (1.35 ± 0.08 ± 0.2)%
10-2
σ (θ ) = σ 1/E
0.012
Delta Theta (rad)
0.07
307
1
0
0.5
(b)
1
1.5
E γ / GeV
2
2.5
3
Fig. 3. (a) Energy resolution for the EMC for photons and electrons, measured from various processes, (b) angular resolution for photons, measured from p0 and Z decays.
5. Resolution The energy resolution is measured with several processes for the different energy regions. The result of a fit to the energy dependence is in good accordance with the data from Monto Carlo including the background (see Fig. 3). The angular resolution is derived from p0 and Z decays into two photons of approximately the same energy. A fit to an empirical parameterization is also is in good accordance with Monte Carlo Simulations.
ity’’ of the In-detector electronics, which can be related to the extensive QC during construction. Only one out of 6580 channels is inoperative due to an inaccessible cable damage. The EMC is performing close to design expectations. The implementation of the digital filtering has significantly improved the energy resolution, which is expected to be improved further with results from ongoing calibration studies.
References 6. Conclusion In the > 2:5 years of BaBar being in factory mode now, we have observed no ‘‘’infant mortal-
[1] D. Boutigny, et al., The BaBar Technical Design Report, SLAC-R-457. [2] B. Aubert, et al., The BaBar detector, BABAR Collaboration, Nucl. Instr. and Meth. A 479 (2002) 1.
The BaBar electromagnetic calorimeter Bernd Lewandowski Institute for Experimental Physics 1, Ruhr-University, Universitaetstrasse 150, D-44780 Bochum, Germany For the BaBar EMC Group
Abstract The BaBar electromagnetic calorimeter is a hermetic, total-absorption array of CsI(Tl)-crystals, operated at the asymmetric e eþ -collider PEP-II at SLAC. The design and the status of the performance as of February 2002 is presented. r 2002 Elsevier Science B.V. All rights reserved.
1. Requirements The BaBar electromagnetic calorimeter is used to measure electromagnetic showers over a wide energy range. The lower boundary is given at the order of 20 MeV to ensure an efficient reconstruction of B-decays containing multiple p0 s; while the upper limit is given by electrons from Bhabha events at 9 GeV; which is increased to 13 GeV to allow for possible running at the Uð5sÞ resonance. The efficient reconstruction of extremely rare decays (e.g. B0 -p0 p0 ) requires an energy resolution at the order 1% [1]. While the p0 mass resolution at lower energies ðo2 GeVÞ is dominated by the energy resolution, at higher energies an angular resolution of a few milliradians is required. The EMC is also the primary subdetector for the separation of electrons and hadrons, which requires a purity at the 0:1% level. To ensure a high efficiency in data taking over the
E-mail address: [email protected] (B. Lewandowski).
10 years lifetime of the experiment, all inaccessible detector elements must be highly reliable.
2. Implementation 2.1. Crystals The EMC is realized as hermetic, total-absorption array of finely segmented scintillating crystals [2]. The material of choice to achieve the design requirements is CsI, which is doped with thallium at the 0:1% level. The high light yield (50,000 g=MeVÞ and small Molie" re radius ð3:8 cmÞ together with the short radiation length ð1:85 cmÞ allow for a relatively compact design of a calorimeter with the required performance. The peak emission at lmax ¼ 565 nm permits the readout of the scintillation light with photodiodes, which allows the EMC to be located completely inside of the solenoid volume to minimize material in front of the crystals. To achieve the required resolution over the whole energy range, the crystals had to pass several steps of quality control
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 1 4 8 4 - 5
B. Lewandowski / Nuclear Instruments and Methods in Physics Research A 494 (2002) 303–307
304
like measurements of light yield and the independence of the light yield from the location of the scintillation along the length of the crystal. 2.2. Geometry The calorimeter consists of a cylindrical barrel and a conical endcap in forward direction, since it is operated at an asymmetric collider. The barrel is segmented in 48 axially symmetric rings of 120 crystals each, while the endcap has 8 rings with 80 crystal in the innermost rings and up to 120 crystals in the outer rings (see Fig. 1). The trapezoidally shaped crystals have a transverse size comparable to the Molie" re radius and a length increasing from 29:6 cm in the backward region to 32:4 cm in forward direction to minimize the shower leakage from increasingly higher energy particles. The 6580 crystals cover a polar angle from 15:81 to 141:81 which corresponds to a solid angle coverage of 90%: 2.3. Readout The scintillation light of the CsI(Tl) crystals is read out with two silicon PIN diodes (Hamamatsu S2744-08) glued to the backward side with a transparent polystyrene coupling plate in between using an optical epoxy to maximize light transmission. The surrounding area of the backside is covered with a white reflective plate to minimize
light loss. The crystals are wrapped in two 165 mm thick layers of white diffuse reflector (TYVEK), 25 mm aluminum foil and 13 mm Mylar foil for insulation. The aluminum housing for a pair of preamplifiers directly connected to the photodiodes is mounted on the back side, connected to the aluminum foil using an electrically conductive glue to provide a Faraday shield. To assure the reliability of all components for the 10-year lifetime, extensive studies like aging tests of the gluejoints have been done and each assembly step was accompanied by several QC tests. 2.4. Electronics The readout electronics for the BaBar EMC is located in three different sections. The first one is the In-Detector region containing all parts, which will be inaccessible after the installation of the calorimeter into the BaBar detector. Each photodiode is read out with a charge-sensitive, low noise preamplifier realized as full custom ASIC design. The signal shaping is implemented as a three stage filter CR-RC-RC with shaping times of 800–250– 250 ns: To achieve the required energy resolution, the electronics is realized as a multi range system. Therefore each preamplifier provides 1 and 32 gain outputs for each diode A and B per crystal. The main amplification and the digitization electronics takes place on the ADC-boards (ADB), which are located in the On-Detector 2359
1555
1375 920
2295
1127
38.2˚
1801
558
Interaction Point
External Support
1979
26.8˚
22.7˚
15.8˚
1-2001 8572A03
Fig. 1. Top half cross-section of the EMC, the detector is axially symmetric around the z-axis (all dimensions in mm).
B. Lewandowski / Nuclear Instruments and Methods in Physics Research A 494 (2002) 303–307
305
region being accessible for system maintenance. On the ADB the Custom Auto Range Encoding Chip (CARE) combines the two signals to an average of A and B for both gains before further amplification to provide four signals of gain 1; 4; 32; 256: Using a comparator circuit, the signal range is identified and encoded in two range bits. The corresponding signal is automatically selected with an analog multiplexer and input into a 10-bit 3:7 MHz ADC. This encoding scheme reduces the dynamic range on the data transmission from 18-bit to 13-bit. On the IO-Board (IOB) the digital data of 24 channels are send across a 30 m long optical fiber to the Read Out Modules (ROM) located in the Off-Detector region, where the continuously digitized data stream is analyzed by the signal processing algorithm called Feature-Extraction (FEX).
backs. The On-Detector electronics is cooled with chilled water pumped through channels milled into the end flanges.
2.5. Mechanical integration
The signal processing is done by an algorithm FEX. First the pedestal subtraction and the correction constants from the electronics calibration are applied to each sample. The information about pulse height and position were extracted from a polynomial fit to the peak region. This has been improved by using a digital filtering. The frequency decomposition for a typical pulse and the corresponding noise spectrum are measured to derive an optimum set of weights that maximizes the signal-to-noise ratio. This implementation has improved the equivalent noise energy for the electronics noise from 440 to 270 keV:
To minimize the material between and in front of the crystals, they are housed in ‘‘eggcrate’’ structures made of thin walled (300 mm) carbonfiber epoxy composite (CFC). The barrel structure is fabricated in 280 separate modules of 7 3y f crystals. After installation of the preamplifier housings on top of the inserted crystals, these aluminum cases are glued to the carbon fiber walls using a thermally conductive epoxy to dissipate the 100 mW load from the preamplifiers. The entire module is equipped with an aluminum strongback, which is also thermally connected. This strong-back allows the handling of the 100 kg package in all directions and is used to mount and align the 280 modules in the main support structure. This aluminum support cylinder carries in the end-flanges in forward and in backward direction minicrates, which are housing the OnDetector electronics. The inner and the outer diameter of the barrel are RF shielded with a double layer of aluminum plates. They also provide a gas seal, allowing the slightly hygroscopic crystals to reside in a dry nitrogen atmosphere. To keep the temperature inside the calorimeter stable at 2070:51C; an active FLOURINERT cooling uses pipes running along the barrel, which are connected to the strong
3. Signal processing and calibration 3.1. Electronics calibration The electronics is calibrated for non-linearities in the front end electronics. A well-defined charge is injected into the input of the preamps, which is done for all four gain ranges separately with overlaps in between to match the ranges. For this procedure, each gain is selected and the range switching disabled. 3.2. Feature extraction
3.3. Source calibration The translation of the measured scintillation light to the deposited energy requires an absolute energy calibration. The single crystal calibration is done at two energies at opposite ends of the dynamic range, requiring one single point o10 MeV at the low-energy side. Therefore, a radioactive source has to be placed in front of each crystal. For the BaBar EMC a fluid source calibration system has been realized. The fluid FLOURINERT is activated in a bath using a neutron generator producing 4 108 n=s: These neutrons are used to activate the isotope 19 F: Using the decay chain 19 F-16 N þ a; 16 N-16 On þ b;
B. Lewandowski / Nuclear Instruments and Methods in Physics Research A 494 (2002) 303–307
On -16 O þ g; 6:13 MeV photons are produced. The activated fluid is circulating through thin manifolds, which are placed between the DIRC and the EMC. The half-life of 7 s of the activated 16 N prevents a long-term activation of this calibration system, but still provides a typical photon rate of 40 Hz=crystal: At this rate a 30 min run performed weekly is sufficient to keep track of changes in the light yield due to radiation damage. 16
4. Monitoring 4.1. Lightpulser A Lightpulser system is used to monitor the complete readout chain of the EMC. The light of a xenon flashlamp (Hamamatsu L4633) is spectrally filtered to match the spectrum of CsI(Tl) scintillation light and distributed to the single crystals through a system of optical fibers. A neutral filter system also allows a continuous variation of the light intensity being used for a linearity check of the complete readout chain. Any pulse-to-pulse variation of the lamp is monitored with a precision reference system. The monitoring is performed daily as a 5 min run including validation. This fast operation guarantees a minimum disruption of data taking.
3.4. Physics events Since the light yield of a crystal is not uniform along its length, calibration points at higher energies are also required. The well-known kinematics of the Bhabha scattering is used for a single crystal calibration. Depending on the polar angle, energy points in the range from 3–9 GeV are available. One day of running at the design luminosity of PEP-II ðL ¼ 3 1033 cm2 =s2 Þ delivers approximately 200 hits per crystal. This is sufficient to realize a calibration with a statistical precision of E0:35%: The p0 calibration is used to correct cluster energies for energy losses from leakage and from interaction with the material surrounding the crystals. The used correction function is a polynomial in lnðEÞ and cosðyÞ; which provides typical correction of the order 6%:
4.2. Radiation damage The accumulated dose of radiation is measured by realtime integrating dosimeters (RadFETs), which are placed in front of the crystals. The size of the observed light loss due to radiation damage (see Fig. 2) is close to expectations, which are based on extensive irradiation tests. Start
Gain HIstory
0 -0.01
450
-0.02 Endcap
-0.03 ∆Gain / Gain
300 Barrel Backward
200
Forward Barrel
10
20 30 40 50 60 Luminosity (fb-1)
70
80
(b)
Jan 2003
0
Endcap Jul 2002
-0.1
Jan 2002
(a)
Backward Barrel
-0.09
Jul 2001
50
-0.08
Jul 2000
100 0
-0.06 -0.07
Barrel Forward
150
-0.05
Jan 2000
250
-0.04
Jan 1999
Dose [Rad]
350
Jul 1999
400
Jan 2001
306
Fig. 2. Radiation monitoring: (a) integrated dose vs. integrated luminosity measured by RadFet’s placed in front of the crystals, (b) relative gain change as measure for radiation damage of the CsI(Tl) crystals.
B. Lewandowski / Nuclear Instruments and Methods in Physics Research A 494 (2002) 303–307 π0 → γγ η → γγ Bhabhas χ c → J/ψ γ radioakt. Source MonteCarlo+BG MonteCarlo
0.06
Delta E / E
0.05
0.014
0.04 0.03
σ (E)/E = σ1/E
0.02
σ 1 = (2.30 ± 0.03 ± 0.3)%
0.01
1/4
⊕ σ2
(a)
η → γγ
σ 1 = (4.16 ± 0.04) mrad
MonteCarlo+BG
σ 2 = (0.00 ± 0.00) mrad
0.01
MonteCarlo
0.008 0.006
0 Eγ / GeV
π0 → γγ
+ σ2
0.002
0 10-1
1/2
0.004
σ 2 = (1.35 ± 0.08 ± 0.2)%
10-2
σ (θ ) = σ 1/E
0.012
Delta Theta (rad)
0.07
307
1
0
0.5
(b)
1
1.5
E γ / GeV
2
2.5
3
Fig. 3. (a) Energy resolution for the EMC for photons and electrons, measured from various processes, (b) angular resolution for photons, measured from p0 and Z decays.
5. Resolution The energy resolution is measured with several processes for the different energy regions. The result of a fit to the energy dependence is in good accordance with the data from Monto Carlo including the background (see Fig. 3). The angular resolution is derived from p0 and Z decays into two photons of approximately the same energy. A fit to an empirical parameterization is also is in good accordance with Monte Carlo Simulations.
ity’’ of the In-detector electronics, which can be related to the extensive QC during construction. Only one out of 6580 channels is inoperative due to an inaccessible cable damage. The EMC is performing close to design expectations. The implementation of the digital filtering has significantly improved the energy resolution, which is expected to be improved further with results from ongoing calibration studies.
References 6. Conclusion In the > 2:5 years of BaBar being in factory mode now, we have observed no ‘‘’infant mortal-
[1] D. Boutigny, et al., The BaBar Technical Design Report, SLAC-R-457. [2] B. Aubert, et al., The BaBar detector, BABAR Collaboration, Nucl. Instr. and Meth. A 479 (2002) 1.